Usami was a member of S4 or Shooting Star Subaru-Sama, and is also a member of the crafts club. She is shown to have considerable leg power which she uses in the form of kicks. She is shown to be a good cook, partly because she lives on her own and has to cook her own meals. She blackmailed Kinjirō, with the picture of him and Subaru on a mock date, to be her boyfriend up until the end of their school fair. Kinjirō accepted because of the condition that she will not spread the photo with Subaru and Kinjirō going out together. However she later becomes friends with Kinjirō and later falls in love with him. Her name is noted to sound similar to the Japanese word for rabbit (ウサギ - Usagi) which becomes the source of Kinjirō's nickname for her; Usamin, her own nickname for him is "stupid chicken". She later allows Kinjirō to call her by her first name, Masamune.
As a gag, Usami is shown to be able to attract ghosts if not living together with them perfectly (ghosts appear in her photos and randomly pops out in her apartment).
In manga, her present occupation is House of Suzutsuki's maid.
Usami shares many traits with Taiga Aisaka from Toradora! Both have a short stature with long hair, an aggressive personality, have issues making friends & showing others their true self, refer to the male lead by a degrading name, and have names reminiscent of animals.